This invention relates to an apparatus for denitration which can remove the nitrogen oxides (hereinafter referred to as NOx) in the combustion offgases discharged from combustion facilities such as industrial furnaces, a variety of boilers, gas turbines, waste treatment equipment, etc.
Recently, because of the limited supply of heavy oil, thermal power plants have been switching from the mono-fuel combustion of heavy oil to the mono-fuel combustion of coal, in order to reduce dependency on petroleum. Large capacity thermal power plants are now being built for the mono-fuel combustion of coal.
However, since coal is less combustible than petroleum, NOx and unburned carbon are likely to be formed in the offgas. Measures to reduce the NOx include flame splitting, the recirculation of the offgas, two-stage combustion, denitration in the furnace, and slow combustion.
Most coal combustion boilers are not operated continuously at full load. They usually follow a "daily start-stop" operation (hereinafter referred to as DSS) in which the operation is either carried out by loading up and down to 80% load, 50% load, and 25% load, or completely stopped; or they follow a "weekly stop-start" operation (hereinafter referred to as WSS) in which operation continues during the weekdays, when there is a great demand for power, but is stopped on the weekends. Thus, in effect, most thermal power plants run at a medium load.
Other styles of medium load power generation include the so-called combined plant, in which the thermal power generation boiler is used in combination with a gas turbine. This system has an exhaust heat recovering boiler and excellent starting characteristics. The plants of such a system follow the above-mentioned DSS or WSS operations.
But even though such systems are improvements over the conventional techniques, their discharge concentration of NOx still exceed the ever-stricter limits dictated by government regulations.
There are an increasing number of plants which have a denitration apparatus of a dry contact reduction system, in which the reaction is carried out in the presence of a catalyst using ammonia (hereinafter referred to as NH.sub.3) as a reducing agent.
FIG. 21 (to be mentioned later) is a system diagram of a conventional denitration apparatus of this kind. In the figure, an inlet flue 1, a denitration reactor 2, an ammonia-injecting means 6, an offgas source (such as combustion equipment, etc.) 7, an air heater 8, a ventilator 9 and a flue 10 are illustrated.
The offgas generated by combustion is vented from an offgas source 7 through an inlet flue 1 which leads to a denitration reactor 2. On the way, an ammonia-injecting means 6 is provided, which injects NH.sub.3, a reducing agent for the NOx in the offgas. The NOx in the offgas reacts with the NH.sub.3 by virtue of the catalyst contained in the denitration reactor 2 and decomposes into harmless water vapor and nitrogen gas.
In order to restrict the discharge of the unreacted portion of the NH.sub.3 used, (that is, to restrict the discharge amount of leaking ammonia), the amount of NH.sub.3 injected is restricted. Such restriction is required because of the fact that when the unreacted portion of the NH.sub.3 is vented from the denitration reactor 2, it reacts with the S0.sub.3 in the gas; as a result, acidic ammonia sulfate adheres to the machinery or instruments downstream of the denitration reactor 2. This has the undesirable effect of lowering the efficiency of the heat exchange.
FIG. 22 (to be mentioned later) is a diagram showing the relation between the amount of ammonia injected (represented in terms of the molar ratio of ammonia to NOx) and denitration performance.
As shown in this figure, when m.sub.1 is employed as the molar ratio in the operation, a denitration rate of .eta..sub.a is obtained. Of the amount of ammonia injected (a+b), (b) represents the amount consumed in the process of denitration, and the remainder, (a), is vented as leaking ammonia from the denitration reactor. Now, if the molar ratio is increased to m.sub.2, the denitration rate can be raised to .eta..sub.b. However, as the molar ratio is increased and the denitration rate approaches its ceiling, the amount of leaking ammonia (A) also increases to an undesirable degree.
In conventional plants, the injection molar ratio is usually restricted so as to make the concentration of the NH.sub.3 discharged as low as possible, in consideration of its effects upon the machinery and instruments downstream of the denitration reactor. It is supposed that the denitration reaction proceeds by the contact of the NH.sub.3 and the NOx absorbed on the catalyst surface; therefore, if the injection molar ratio is restricted, the amount of NH.sub.3 absorbed on the catalyst surface inevitably decreases, as compared to when the injection molar ratio is high. The denitration reaction cannot proceed efficiently on the whole catalyst surface. This was the problem encountered when considering the most efficient use of the catalyst.